Biophysical Insights into the Role of Amyloid-Beta Misfolding in Alzheimer’s Disease Pathogenesis.

Abstract

Amyloids are protein aggregates that build up as plaques in various tissues in the body and are associated with a number of diseases. Of the amyloidoses, Alzheimer’s disease (AD) is the most known and socially distressing. Amyloid-beta (Abeta) is the amyloidogenic protein associated with AD and is implicated in the etiology of the disease. Abeta aggregation is highly heterogeneuos, giving rise to a number of possible aggregation pathways and intermediate oligomeric structures. The mechanism of Abeta aggregation was studied here in the presence and absence of a model cell membranes employing fluorescence spectrosopy, light scattering, atomic force microscopy, and NMR spectroscopy. First, Abeta aggregation is investigated in the presence of a lipid bilayer, exploring the particular role of lipid composition on the mechanism of membrane disruption. It was shown that membrane disruption by Abeta occurs by a two-step process: (i) intial formation of ion-selective pores followed by (ii) non-specific fragmentation of the lipid membrane during amyloid fiber formation. Moreover, the presence of gangliosides enhances pore formation and is necessary for fiber-dependent membrane fragmentation. Next, magic angle spinning (MAS) NMR is used to gain structural insights on an Abeta oligomer, providing atomic-level characterization on a non-fibrillar product of Abeta. Importantly, it is demonstrated that MAS NMR and 1H-1H dipolar interactions can be used as a spectral filter to detect Abeta oligomers without a purification procedure. In comparison to other solid-state NMR techniques, the experiment is extraordinarily selective and sensitive, as it can resolve spectra on a small population of oligomers (7% of the total Abeta concentration). Using this method, it was shown that a stable, primarily disordered Abeta oligomer forms and coexists with amyloid fibers. Finally, a real-time 2D NMR method is implemented to study the mechanism of Abeta fiber elongation. It is demonstrated that monomeric Abeta undergoes a conformational conversion after binding to the fiber surface to complete the elongation step, with the strongest interaction occurring in the central region of the peptide (residues Phe19- Glu22). To our knowledge, this is the first high-resolution account of the fiber elongation process and provides residue-specific details of amyloid fiber polymorphism.